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September 28, 2020 – Dry, windy, and hot conditions will lead to critical fire weather conditions through today. Above normal temperatures and dry conditions will linger through the second half of the week.

Discussion

Offshore wind event continues to bring critical fire weather conditions to northern California. Presently observing surface pressure difference of +12.6 mb from Reno to Sacramento, indicative of the east-west offshore gradient. Yesterday at this time it was only +7.1 mb. Remarkable winds have been realized — Jarbo Gap in Butte County is gusting to 58 mph with a relative humidity value of 13 percent. These exceptionally critical fire weather conditions will continue overnight for the region, with the area of greatest concern in the east-west oriented canyons, gaps, and passes in the northern Sierra and foothills. Northerly winds continue to impact the western side of the Sacramento Valley while the north-south gradients have improved marginally (present Medford to Redding surface pressure gradient of 6.0 mb; peaking earlier on Sunday at 9.9 mb).

Forecast models have winds beginning to gradually subside after sunrise Monday, but relative humidity values will remain very low as temperatures soar into the 90s to near 100 deg F. Red Flag Warning continues through Monday evening. Additionally, a Wind Advisory where the strongest wind are expected — including the northern Sierra foothills and southern Cascades from Butte and Plumas Counties north to eastern Shasta County — will continue through 2 pm PDT today.

Upper level ridge will remain parked over the West Coast through much of this week, resulting in the prolonged period of widespread 90s to low 100s throughout the Sacramento Valley and northern San Joaquin Valley. This equates to about 10 to 15 deg F above late September normals with moderate heat risk.

Extended Discussion (Friday through Monday)

Upper ridging overhead Friday slowly weakens through the weekend as mesoscale offshore upper low approaches California Sunday and lingers along the coast early next week. Above normal high temperatures continue Friday with a gradual cooling trend through the extended forecast period. Highs Sunday/Monday are expected in the upper 80s to low 90s for the Central Valley with mostly 70s to 80s for the mountains and foothills. Dry weather continues.